1. Field of the Invention
The present invention relates to a power supply having a mixed mode regulator, and more particularly to a power supply having a mixed mode regulator that is able to provide advantages such as small size and high sensitivity outputting power source.
2. Description of Related Art
With reference to FIG. 6, a conventional power supply with a series pass regulator comprises a transformer (50), a rectifier and filter circuit (51) connected to the output of the transformer (50), and a comparator (52) connected through a transistor Tr1 to an output of the rectifier and filter circuit (51). The rectifier and filter circuit (51) includes a full filter BD1 and a high voltage capacitor C1 connected to an output of the full filter BD1, that rectifies the AC power source to a DC power source.
When the power supply connects a load (not shown) with the AC power source, an input of the comparator (52) connects to a voltage divider R1, R2 connected to the load (not shown). The output voltage of the comparator (52) controls the transistor Tr1 to be in a conduction or non-conducting state, that is, the transistor Tr1 is used to control current of the DC power source to the load. When a loading of the load is increasing, the output of the comparator (52) outputs a positive voltage to the transistor Tr1, and the current of the Tr1 increases. On the other hand, when the output of the comparator (52) outputs a negative voltage to the transistor Tr1, the current from the transistor Tr1 decreases.
As pre the above description, the power supply with the series pass regulator is able to supply the suitable current to the load according to the present load needed. However as the transformer (50) of the regulator is a low frequency type transformer (50) took up large space, the whole size of the power supply is big. Therefore, it is difficult to achieve a small type of a measuring device using the series pass regulator. Furthermore, the transistor Tr1 has to use the powerful transistor Tr1 to eliminate any possible over-voltage in the loop with the load. However, when the powerful transistor Tr1 has been conducting for a while, the DC power source produces undesirable high temperatures, so that the series pass regulator has to form a heat sink to decrease high temperatures from the powerful transistor Tr1.
With reference to FIG. 7, a power supply with a switching mode regulator comprises a rectifier and filter circuit (60), a high frequency transformer (61) connected to an output of the rectifier and filter circuit (60), a decreasing voltage circuit (62) connected an output of the rectifier and filter circuit (60), a comparator (63) connected to an output of the decreasing voltage circuit (62), a photo coupler (64) connected to the output of the comparator (63), and a PWM IC (pulse width modulator) (65) connected to an output of the photo coupler (64) and connected through a transistor Tr2 to a first coil of the high frequency transformer (61). An output of the decreasing voltage circuit (62) is an output of the power supply to connect a load (not shown) needing a DC power source.
An output of the PWM IC (61) is used to control the duty cycle of the transistor Tr2, that is, when a loading of the load is increasing, the duty cycle of the transistor increases to supply more DC power to the load. On the other hand, when the loading of the load decreases, the comparator (63) outputs a high voltage to the PWM IC (65), and then the PWM IC (65) controls the duty cycle of the transistor Tr2 to increase output power. The transistor Tr2 controlled by the PWM IC (65) has two states, conductive state and nonconductive state. When the transistor Tr2 is in the conductive state, the transistor Tr2 is able to decrease the consuming power transmitting through the decreasing voltage circuit (62). The transistor Tr2 is not able to produce the high temperature, and the transistor Tr2 is the high frequency type, so that the switching mode regulator is able to normally applying the power to the load without the heat sink (not shown). That is, the size of the whole regulator is small, and is suitable to be received in the measuring device. However, the transistor Tr2 often unreliably switches to either of the two states, thus the output DC power of the regulator is unstable.
As pre the above description, the power supply uses whether the series pass regulator or the switching mode regulator each regulator has different the drawbacks. However, any precision instrument needs a power supply with a stable output power.
To overcome the shortcomings, the present invention provides a power supply with a mixed regulator to mitigate and obviate the aforementioned problems.
The objective of the present invention is to provide a power supply having a mixed mode regulator, and furthermore, an output of the power supply uses the series pass controlling circuit to increase sensitivity for detecting the loading of the load. A switching voltage mode voltage circuit eliminates the need for a powerful transistor and a heat sink. Therefore the power supply provides a stable power output and allows a compact size of the power supply to be achieved. Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.